Age and Sex Are Critical Factors in Ischemic Stroke Pathology

Abstract

Ischemic stroke is a devastating brain injury resulting in high mortality and substantial loss of function. Understanding the pathophysiology of ischemic stroke risk, mortality, and functional loss is critical to the development of new therapies. Age and sex have a complex and interactive effect on ischemic stroke risk and pathophysiology. Aging is the strongest nonmodifiable risk factor for ischemic stroke, and aged stroke patients have higher mortality and morbidity and poorer functional recovery than their young counterparts. Importantly, patient age modifies the influence of patient sex in ischemic stroke. Early in life, the burden of ischemic stroke is higher in men, but stroke becomes more common and debilitating for women in elderly populations. The profound effects of sex and age on clinical ischemic stroke are mirrored in the results of experimental in vivo and in vitro studies. Here, we review current knowledge on the influence of age and sex in the incidence, mortality, and functional outcome of ischemic stroke in clinical populations. We also discuss the experimental evidence for sex and age differences in stroke pathophysiology and how a better understanding of these biological variables can improve clinical care and enhance development of novel therapies.

This mini-review examines the interactive effects of age and sex on ischemic stroke pathology.

Stroke is a deadly and debilitating disease, affecting >15 million people worldwide each year. Stroke is a disease of aging—most strokes occur in people >65 years (1, 2). Aged patients have higher mortality and poorer quality of life after stroke compared with younger patients (3–8). Sex also affects stroke incidence and outcome; although men have a higher incidence of stroke throughout most of the lifespan, women have higher stroke prevalence overall because of the increase in stroke risk with aging and a longer average lifespan in females (9).

Ischemic strokes, caused by a loss of blood flow to the brain, account for 87% of all strokes in the United States (2). Age and sex influence ischemic stroke epidemiology, pathophysiology, and treatment efficacy. Fascinatingly, one recent clinical study found that minocycline, an inhibitor of cell death, improves functional outcome after ischemic stroke in male patients only, with no benefit to female patients (10). Echoing this, a male-specific benefit of minocycline was also seen in an experimental mouse model of ischemic stroke, illustrating the importance of assessing sex as a variable in both clinical and experimental research (11).

Unfortunately, the consideration of sex as a variable remains critically underassessed in many clinical trials. Notably, the Systolic Blood Pressure Improvement Trail (SPRINT) study reported that aggressive control of blood pressure improves composite cardiovascular risk, including the risk for stroke. Unfortunately, only 36% of the patients enrolled in SPRINT were female, and the study was terminated because of improved outcomes in men [hazard ratio (HR), 0.72; 95% CI, 0.59 to 0.88] before statistical significance was reached in females (HR, 0.84; 95% CI, 0.62 to 1.14) (12, 13). It remains unclear whether the SPRINT trial findings are generalizable to female patients without further study (14). In cases such as this, enrollment and/or follow-up in women should continue until the study is adequately powered to definitively assess the effects of the intervention in both sexes.

Effect of Sex and Age on Clinical Ischemic Stroke

Age-dependent sex differences in stroke incidence, mortality, and morbidity

Sex differences in ischemic stroke epidemiology depend on patient age because the influence of sex on stroke risk and outcome changes across the lifespan. In childhood and early adulthood, males have a higher incidence of ischemic stroke and poorer functional outcomes than do females (9, 15). In middle age, the rates of ischemic stroke begin to increase in women, concomitant with the onset of menopause and loss of female sex hormones (16). After middle age, stroke rates continue to increase in women, with some reports of higher stroke incidence in elderly women (age >85 years) compared with elderly men (9, 15).

Mortality risk in ischemic stroke is high, with estimated 30-day case fatality ranging from 16% to 32% (1, 17). The complex relationship between age and sex complicates the assessment of sex differences in stroke-related mortality. Numerous studies have reported higher mortality in women with ischemic stroke, yet sex differences in age of stroke patients represent major confounders (9, 18, 19). After adjustment for age and prestroke function, no independent effect of sex on stroke mortality risk was observed (19–23). Although female sex may not independently predict mortality, it is important to recognize that the average female stroke patient will be at higher risk for death than will her male counterparts. Therefore, sex stratification of clinical trial results is critical to determining whether the risk/benefit ratio is sex-dependent (as in the minocycline trial) (10, 24).

Because most ischemic stroke patients survive their initial injury, functional outcome in stroke survivors is a major determinant of overall disease burden. There are ~ 6 million stroke survivors living in the United States (3.8 million female and 2.2 million male), and this number is projected to increase to 10 million by 2030 as the aging population expands (2). The economic cost of stroke survivor care increases with age because elderly stroke survivors (age ≥65 years) are more likely to have severe deficits and require greater care. Sex disparities in functional outcome after ischemic stroke are well described, with overwhelming evidence for poorer functional outcomes and a reduced quality of life in female patients (25–31). Importantly, these sex differences persist after adjustment for confounders, indicating that female sex may be an independent risk factor for poor functional outcome and impaired quality of life after ischemic stroke. In addition, elderly women tend to have poorer baseline functional status before stroke, resulting in their frequent exclusion from clinical trials and increasing the likelihood of underestimation in therapeutic benefit (19, 23).

These sex differences in stroke epidemiology are likely due to both socioeconomic and biologic differences. Several socioeconomic risk factors for poor stroke outcomes are significantly more common in women. Poststroke depression occurs in 33% of stroke survivors and is associated with poor functional recovery and increased mortality (32–34). Most studies included in a large meta-analysis found higher rates of poststroke depression in women (35). Female stroke patients are also more likely to live alone than are male patients, which may contribute to a delay in hospital arrival and poor social support during recovery (36). Other factors, including sex differences in recognition of stroke symptoms and clinical care, have been comprehensively reviewed elsewhere (37, 38). In this review, we focus on sex differences in the biology of ischemic stroke, including sex differences in the incidence, timing, and magnitude of risk factors for ischemic stroke.

Age and sex differences in ischemic stroke risk factors

Stroke risk factors are important determinants of the incidence and pathophysiology of ischemic stroke (39). Age itself is a significant, nonmodifiable risk factor for ischemic stroke. Because women tend to be older than men at stroke onset, sex differences in stroke incidence and stroke outcomes may be partially explained by age-dependent differences in the modifiable risk factor profile of male and female patients (19, 25, 40). Some of these modifiable risk factors (Table 1) are sex-specific, whereas others occur in both men and women but have a higher incidence or confer a higher overall stroke risk in one sex (69). Interestingly, sexually dimorphic risk factors change throughout the lifespan, contributing to the shifting sex differences in stroke incidence, as depicted in Fig. 1.

Table 1.

Summary of Current Knowledge Regarding Sex Differences in Risk Factors for Ischemic Stroke

Risk Factor	Type	Risk Magnitude and Modifiers (Reference)
Pregnancy	Sex-specific (female)	Low overall (34:100,000 deliveries) (41)
		Highest risk postpartum (up to 12 wk) (42)
		Elevated risk in preeclampsia and gestational hypertension (41, 43)
OCP	Sex-specific (female)	Increased risk (1.4-fold to 2-fold) in women (44)
		Higher risk correlates to higher dose of estrogens (45)
		Synergistic risk in women who have migraines with aura (46–48)
Postmenopausal HRT	Sex-specific (female)	Late HRT shows no benefit or increases stroke risk (49–52)
		Evidence for benefit of early HRT (<6 y from menopause onset) in cardiovascular disease, stroke risk still in follow-up (53)
AF	Shared	Ischemic stroke risk conferred by AF increases with age (54, 55)
		Prevalence of AF and cardioembolic stroke higher in women; women tend to be older at onset (56–60)
		AF confers stronger stroke risk in women (61)
Metabolic syndrome	Shared	Metabolic syndrome confers stronger stroke risk in women (62)
		Risk factor profile of women with metabolic syndrome dominated by abdominal obesity, which is suggested to confer greater stroke risk in women (63)
Diabetes	Shared	Type 2 diabetes confers greater risk for stroke in women (64)
		Risk for fatal stroke higher in diabetic female patients than male patients (65)
Hypertension	Shared	Hypertension is more common in women and more likely to be poorly controlled (66)
		Stroke risk conferred by hypertension equivalent between men and women (67)
Coronary artery disease	Shared	Coronary artery disease is more prevalent in men (16, 68)

Sexually dimorphic risk factors are identified as sex-specific (pregnancy, OCP use, postmenopausal HRT) or shared (metabolic syndrome, diabetes, hypertension, coronary artery disease). Current evidence for the overall risk, any risk modifiers or synergistic relationships, and any sex differences in the risk magnitude are described with pertinent citations given.

Abbreviations: AF, atrial fibrillation; HRT, hormone replacement therapy; OCP, oral contraceptive pill.

Figure 1.

Schematic illustration of the relationship between age- and sex-related risk factors across the lifespan. An approximation of the overall increase in stroke incidence with aging is given, in addition to sex-specific or sexually dimorphic stroke risk factors at each stage of life. Sex differences in stroke incidence throughout the lifespan are also depicted. AF, atrial fibrillation; MetS, metabolic syndrome; OCP, oral contraceptive pill.

Women have several unique stroke risks, including oral contraceptive pill (OCP) use, pregnancy, menopause, and hormone replacement therapy (HRT). Although stroke incidence is low in women of reproductive age, OCP use significantly increases stroke risk, with the highest risk conferred by high-estrogen OCPs (44). Estrogens have many positive cardiovascular effects (as discussed later), but they also enhance coagulation and may elevate the risk for clotting in women taking estrogen-containing OCP (41, 70). Interestingly, there is a synergistic stroke risk in women taking OCPs with a history of migraines, particularly in patients who experience migraine auras (42, 43).

Stroke incidence in women also increases during pregnancy, with particularly high risk during the final trimester and early postpartum period (45–48). Stroke risk is further increased in women who develop gestational hypertension and preeclampsia during pregnancy (49, 50). Rates of preeclampsia are higher with advanced maternal age, with risk increasing from 6.4% to 9.4% in primiparous pregnant women >35 years (51). Importantly, although preeclampsia is often thought to be a disorder of primiparity, several studies have shown that advanced maternal age increases the risk for preeclampsia even in multiparous women (52–56). Although the short-term consequences of preeclampsia, including acute stroke and mortality, are well known, data on the long-term complications of preeclampsia are also beginning to emerge (46, 55, 56). Women with a history of preeclampsia have a greater risk for future cerebrovascular disease, with more frequent and severe white matter lesions in the brain out to 18 years after pregnancy (57). A meta-analysis of 6.4 million women confirmed that preeclampsia is significantly associated with long-term cardiovascular disease risk, including a twofold increase in stroke risk later in life (58). Fascinatingly, evidence also supports long-term effects in children exposed to preeclampsia in utero, who experience higher blood pressure and increased body mass index later in life (59–62). Although studies of long-term outcomes are rare, Kajantie et al. (63) reported an increased risk for stroke in children of mothers with both gestational hypertension (HR, 1.4; 95% CI, 1.0 to 1.8) and preeclampsia (HR, 1.9; 95% CI, 1.2 to 3.0). The mechanism behind this increased risk remains unknown, but preeclampsia alters endothelial function in both mother and her offspring, potentially via alterations in miRNA, aberrant vessel development, and preeclampsia-induced changes in epigenetic gene regulation (64–66).

Although men have higher ischemic stroke rates throughout most of adulthood, ischemic stroke incidence increases in middle-aged women secondary to menopause and the resulting decline in female sex hormones. The onset of early menopause in women (<42 years) is associated with a twofold increased risk for ischemic stroke, even after adjustment for age and other cardiovascular risk factors (67, 68, 71). Several randomized clinical trials were designed to examine whether HRT could reduce the incidence or severity of ischemic stroke in postmenopausal women. Unfortunately, two of these trials found no benefit of HRT after menopause on ischemic stroke rates (72, 73). Two additional trials found that postmenopausal women receiving HRT were at increased risk for ischemic stroke and experienced more severe strokes compared with women in the placebo arm (74, 75).

One potential reason for the paradoxical failure of these HRT clinical trials was the timing of hormone replacement—many patients did not begin receiving HRT until well after the onset of menopause (76). Subsequently, the Early Versus Late Intervention Trial with Estradiol trial was designed to examine the importance of timing in HRT therapy by comparing outcomes in women receiving early HRT (initiated <6 years from the onset of menopause) with those among women receiving late HRT (initiated ≥10 years from menopause onset) (77). Although the follow-up period is still insufficient to assess the effects of HRT timing on stroke risk, early results from the 5-year follow-up study indicate that early, but not late, HRT significantly reduced the progression of cardiovascular disease (77). In support of this, a recent experimental study found that the timing of HRT in a mouse model of stroke significantly affected stroke outcome; aged female mice receiving early 17β-estradiol replacement had significantly improved stroke outcomes compared with females that received delayed HRT (78). These results demonstrate that patient age at menopause and the timing of HRT replacement therapy after menopause may play a critical role in ischemic stroke risk and the efficacy of HRT replacement.

In addition to sex-specific risk factors, sexual dimorphisms exist in shared ischemic stroke risk factors (79). Increased ischemic stroke incidence in the aging population is largely driven by an increased prevalence of these risk factors, including atrial fibrillation, obesity, type 2 diabetes, hypertension, hypercholesterolemia, and coronary artery disease (39). Interestingly, certain common risk factors are more likely to give rise to the different major subtypes of ischemic stroke (large-vessel disease, small-vessel disease, and cardioembolic stroke). Small- and large-vessel strokes result from narrowing or thrombus formation within the cerebral arteries themselves, whereas cardioembolic strokes result from clots formed in the heart (80). As a result, clots of cardioembolic origin tend to occlude the larger cerebral arteries, resulting in greater tissue damage and poorer outcomes (29).

Atrial fibrillation (AF) is a major risk factor for ischemic stroke, accounting for 45% of all cardioembolic-subtype strokes (79). Furthermore, age is a major risk factor for AF, and the stroke risk conferred by AF increases with age (81, 82). The prevalence of AF and cardioembolic ischemic stroke are significantly higher in women than men, likely driven in part by the fact that women tend to be older at the time of stroke onset (83–87). Having AF confers a greater stroke risk in women than in men, and women with AF tend to experience more severe strokes than their male counterparts (88, 89). Because cardioembolic strokes have been linked to larger strokes, higher mortality, and poorer functional outcomes, the increased incidence of AF and cardioembolic strokes in women may contribute to sex differences in mortality and functional outcome after ischemic stroke, particularly in elderly populations.

In recent years, studies have reported a link between ischemic stroke incidence and a common constellation of risk factors known as the metabolic syndrome (90–92). Clinical metabolic syndrome is characterized by the presence of hyperglycemia, central adiposity, hypertension, increased triglyceride levels, and low high-density lipoprotein cholesterol (93). A large meta-analysis recently found that the magnitude of stroke risk associated with metabolic syndrome is significantly higher in female patients than male patients (92). Although men tend to have a more heterogeneous combination of metabolic syndrome risk factors, the risk factor profile of women with metabolic syndrome tends to be dominated by abdominal obesity, as defined by waist circumference ≥88 cm (men) or ≥102 cm (women) (94). Abdominal obesity is a combined measure of both increased subcutaneous fat as well as increased visceral (intra-abdominal) fat. Rates of overall abdominal obesity, particularly visceral adiposity, increase significantly around the time of menopause (95–100). Interestingly, whereas women tend to have lower visceral fat mass and higher subcutaneous fat mass than men in younger groups, levels of visceral adiposity in women increase to equalize with those of men in older populations (100, 101)

Several studies suggest that overall abdominal obesity confers greater stroke risk in female patients compared with male patients (16, 102, 103). Recent work has shown that adipose-resident CD8 T cells, which contribute to adipose tissue inflammation and obesity-induced insulin resistance, are more prevalent and exhibit more activated proinflammatory phenotypes in middle-aged women than in men (104–106). Loss of estrogens in middle-aged women and underlying chromosomal sex (XX) are both believed to drive this increase in abdominal adiposity, adipose inflammation, and insulin resistance (107, 108). Furthermore, increased abdominal obesity in women at menopause has also been linked to an elevated risk for insulin resistance and type 2 diabetes (109).

Diabetes is also a strong risk factor for stroke incidence, recurrence, and stroke-related mortality (110–114). Importantly, a recent large-scale meta-analysis demonstrated that type 2 diabetes confers greater risk for stroke in women (relative risk, 2.28; 95% CI, 1.93 to 2.69) than in men (relative risk, 1.83; 95% CI, 1.60 to 2.08) (115). The risk for fatal stroke is higher in female diabetic patients than in male patients, and a recent study reported that diabetic women are at significantly higher risk for 5-year mortality after stroke than nondiabetic women, a risk that relationship that was not seen in men (116, 117). Overall, diabetes represents a stronger risk factor for stroke in women than in men, and female diabetic patients have poorer outcomes after stroke than their male counterparts. The driving factors behind this sex disparity remain largely unknown, although both epidemiological (delayed diagnosis, undertreatment) and biological (exacerbated endothelial dysfunction, elevated proinflammatory signaling) mechanisms have been suggested (116).

In conclusion, age and sex have an interactive effect on ischemic stroke risk, incidence, and outcome. Studying the mechanisms and consequences of this interaction are critical to the development of safe and effective treatments for stroke patients of both sexes. In the next section, we discuss the current experimental evidence regarding the effect of sex and age on ischemic stroke pathophysiology and therapeutic response.

Age and Sex Differences in the Pathophysiology of Experimental Ischemic Stroke

With the exception of thrombolytic drugs, no pharmacologic therapies have been approved for ischemic stroke (118). Although countless compounds have shown promise in preclinical studies, these protective effects have failed to translate into clinical efficacy for patients (119). This failure may be due, in part, to the use of young male animals in most preclinical studies (119, 120). Although studies comparing ischemic stroke in male and female animals are limited, it is clear that sex-specific responses to cerebral ischemia can be replicated in vivo and in vitro. Furthermore, these results highlight the critical importance of considering age and sex into account when determining therapeutic efficacy in both preclinical studies and clinical trials.

A wealth of evidence has shown that estrogens play protective roles throughout many tissues, including the brain, adipose tissue, heart, and vasculature (121, 122). In the context of ischemic stroke, estrogens are highly neuroprotective at multiple levels, including suppression of stroke risk factor pathology via antiatherogenic effects in the vasculature and the regulation of adipogenesis. Estrogens also ameliorate stroke pathology itself via vasodilation of the coronary arteries and direct neuroprotection of brain and glial cells during ischemia (123). Preclinical studies have also indicated a role for progestins (specifically progesterone) in neuroprotection after ischemic stroke, as reviewed comprehensively elsewhere (124, 125). Interestingly, stroke incidence and severity are higher in males before the onset of puberty, indicating that biological (chromosomal) sex may also be an important determinant of ischemic stroke risk and pathophysiology (126, 127). In this section, we highlight basic science findings that indicate a role for both hormonal and chromosomal sex in the biology of ischemic stroke.

Evidence for sex and age differences in experimental models of ischemic stroke

In vitro stroke modeling is performed by using oxygen glucose deprivation (OGD), in which neuronal, glial, or mixed cells are cultured without glucose in deoxygenated conditions, mimicking the acute energy failure after loss of cerebral blood flow (128). OGD causes greater damage to male-derived neurons, endothelial cells, and astrocytes compared with those from females (129–132). Small in vivo rodent models are also used to model ischemic stroke and to test potential therapeutic strategies. Studies of sex differences in experimental rodent models show that female animals are protected against cerebral ischemia compared with males (133). Importantly, this advantage disappears after ovariectomy, indicating that estrogens are critical to the ischemic resistance seen in females (134–137).

As in clinical ischemic stroke, sexual dimorphism in ischemic stroke is highly dependent on age. In middle-aged mice (age 16 months), the ischemic protection phenotype actually reverses, resulting in larger infarcts and poorer outcomes in female animals compared with males (138). Because this age coincides with the onset of reproductive senescence in mice, increased ischemic sensitivity in middle-aged females may be driven by the recent loss of estrogens, mirroring the effects seen in ovariectomized young females (133, 134). Supporting this theory, and echoing results in patients, early replacement of 17β-estradiol in middle-aged animals has been found to reduce stroke size in mice of both sexes (138). In elderly male and female mice, which have similar sex hormone levels, stroke size equalizes (139). These results show that sex and age also have an interactive effect on experimental stroke pathophysiology, in addition to their influence on clinical stroke.

During ischemic stroke, loss of blood flow to the brain results in two stages of damage. The primary stage is characterized by mitochondrial disruption, energy failure, and ionic imbalances, causing rapid and irreversible cell death (140, 141). Neuronal and glial cell death results in the extracellular release of excitatory neurotransmitters, reactive species, and inflammatory factors that contribute to secondary damage pathways, including excitatory neurotoxicity, oxidative damage, and inflammation (142–144). A schematic overview of the current knowledge regarding sex differences in experimental stroke pathophysiology and treatment efficacy is given in Fig. 2.

Figure 2.

Schematic representation of sex differences in stroke pathology and therapeutic efficacy. Sex differences in mitochondrial disruption exist; experimental stroke models in both mice and rats have shown that female animals experience less oxidative stress and dysregulated respiration after ischemia than males, an advantage that is eliminated by ovariectomy or aging. Sex differences in vasodilation have also been described, in which experimental mouse models have shown that females experience enhanced vasodilation at baseline compared with males, and male animals show benefit only when vasodilation is enhanced [inhibition of soluble epoxide hydrolase (sEH), selection of angiotensin II type 2 receptor (Agtr2)]. Studies in mice have shown enhanced reactive astrocytosis in females after ischemic, which can be ameliorated by the deletion of the proinflammatory protein Pannexin 1. Conversely, female mice show reduced microglial activation compared with male animals in experimental mouse models of ischemic stroke. In vivo and in vitro studies in mice and mouse-derived neuronal cells have demonstrated that the PARP-1/nitric oxide (NO) pathway of cell death predominates in males, whereas the caspase pathway of cell death is dominant in females. In line with this, inhibition of each of these pathways benefits only males, or only females, respectively.

Sex differences in neuronal dysfunction and cell death

Primary ischemic damage includes the disruption of neural cell mitochondria, which are critical to maintaining cellular energy and regulating oxidative stress (141). Both estrogens and progestins enhance mitochondrial respiration and augment antioxidative processes in neurons (145). Female animals have more efficient oxidative respiration than males overall, even after ischemic stroke (146, 147). Ovariectomy in females impairs mitochondrial respiration and antioxidant capabilities, which can be reversed by the addition of exogenous 17β-estradiol (146, 148–153).

Mitochondrial disruption and energy failure will ultimately result in the activation of cell death pathways. Striking differences exist in ischemic-induced cell death between males and females. Male cell death is dominated by the formation of nitric oxide and activation of poly-ADP ribose polymerase 1 (PARP-1), causing mitochondrial depolarization and cell death (154). Inhibition of PARP-1 and nitric oxide in experimental ischemic stroke is protective only in male animals, with no benefit to females (11, 133). Conversely, caspase-inhibiting therapies are preferentially protective in female animals after experimental stroke, as cell death in females largely depends on ischemia-induced activation of caspases (155–157). This sexual dimorphism in cell death pathways persists in females after ovariectomy, indicating that sex hormones are not solely responsible for the sexual dimorphism seen in cell death (155). Chromosomal sex may contribute to this sex difference, as female neurons (XX chromosomes) exhibit greater activation of the caspase pathway compared with male neurons (XY) after OGD in culture (131).

Sex differences in endothelial and glial responses to ischemia

In addition to findings in neuronal cells, sexually dimorphic responses are also seen in other central nervous system cell populations after ischemia, including cerebral endothelial cells, astrocytes, and microglia.

Endothelial cells are major regulators of cerebral blood flow and play a critical role in ischemic injury. Ischemic protection in young females is mediated in part via sex differences in endothelial cell function. As in neurons, endothelial cells derived from male rodent brains exhibit greater sensitivity to OGD than female endothelial cells (132). Males have higher levels of brain soluble epoxide hydrolase (sEH), a suppressor of vasodilation, compared with females (158). Pharmacologic inhibition of sEH reduces ischemic sensitivity in male endothelial cells, and targeted in vivo deletion of sEH improves cerebral blood flow and infarct size in male animals only, with no effects in females (132, 158). Deletion of another vasodilatory mediator, angiotensin II type 2 receptor (AGTR2), exacerbates infarct size and worsens cerebral blood flow in male mice, but not in females (159). Ovariectomy did not worsen female outcomes, suggesting that the sex difference in AGTR2, which is expressed on the X chromosome, may be mediated by chromosomal sex differences (159).

Astrocytes are critical to the health of the cerebrovascular system, assisting in restoration of the blood-brain barrier and scar formation after brain injury (160). Female astrocytes are resistant to OGD-induced ischemic damage, a protective effect that depends on estrogens (161). Sex differences have also been described in glial fibrillary acidic protein, a protein critical to scar tissue formation that is highly expressed in astrocytes (160). After ischemia, female mice have an augmented upregulation of astrocytic glial fibrillary acidic protein compared with that in males (162). Knockout or inhibition of another astrocytic protein known as Pannexin 1, an initiator of astrocytic inflammation, is uniquely protective in middle-aged female animals after ischemic stroke, with no effect in males (163).

Cerebral ischemia also induces massive activation of microglia, the resident immune cell of the central nervous system (164, 165). Immune activation after ischemic is critical to both secondary tissue damage and tissue repair and regeneration (142). Sex differences in ischemia-induced inflammation have been reported at both the hormonal and the chromosomal level (78, 166–168). Early in life, at the neonatal and young adult stages, male animals have higher ischemia-induced microglial activation than do female animals (169, 170). This anti-inflammatory phenotype in females may be driven by female sex hormones, as in vitro studies show that estrogens and progestins can suppress the release of inflammatory mediators in microglia and reduce neuronal cell death (171–173).

Conclusion

Sex and age have a complex and substantial influence on ischemic stroke risk, outcome, and pathology. The high burden of stroke in men throughout early life reverses in middle-age, with increased stroke risk, mortality, and poor functional outcome seen in elderly women. Clear sex differences exist in stroke risk factor profiles, particularly in the incidence and risk conferred by atrial fibrillation and the metabolic syndrome. Although many of these differences are mediated by age-related changes in sex hormones, evidence from experimental studies suggests that chromosomal sex also affects ischemic stroke pathophysiology. Most importantly, this review highlights the critical importance of considering sex and age as biological variables. This can be achieved by (1) designing future clinical research to ensure that studies are adequately powered to assess the effects of sex and age on therapeutic outcome and (2) the inclusion of young and aged animals of both sexes in studies of ischemic stroke pathophysiology and in studies using novel therapeutics.

Glossary

Abbreviations:

AF

atrial fibrillation

Agtr2

angiotensin II type 2 receptor

HR

hazard ratio

HRT

hormone replacement therapy

OCP

oral contraceptive pill

OGD

oxygen glucose deprivation

PARP-1

poly-ADP ribose polymerase 1

sEH

soluble epoxide hydrolase

SPRINT

Systolic Blood Pressure Improvement Trail